All issues / Volume 8 (2014) / Issue 10 (October)
This is an editorial article. It has no abstract.
New conjugated PPV derivatives containing the chiral isosorbide group (P1-3) have been synthesized via the Gilch reaction. The polymers are optically active, soluble in common organic solvents and show good film-forming abilities. High number-average molecular weights were determined by size exclusion chromatography (SEC) (16•103–21•103 g•mol–1). The molecular structures of the polymers were confirmed by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies. Thermogravimetric analysis of the polymers showed good thermal stability up to 320°C. The optical properties of these π-conjugated materials were investigated by UV-vis absorption and photoluminescence (PL) spectroscopies. The polymers show a yellow fluorescence in dilute solution, and an orange emission is observed in thin films. The introduction of the polar isosorbide groups improved the PL intensity, and quantum yields between 50 and 73% were obtained. The HOMO-LUMO energy levels were estimated by cyclic voltammetry, and the electrochemical gaps were 1.81, 1.83 and 2.48 eV for P1, P2 and P3, respectively. Single-layer diode devices were fabricated and show relatively low turn-on voltages between 3.1 and 3.4 V.
Epoxy nanocomposites with commercial carbon nanotubes (CNT) or graphene (GN) have been prepared using phosphonium ionic liquid [trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate, IL-f]. IL-f served simultaneously as nanofiller dispersing medium and epoxy resin catalytic curing agent. An influence of IL-f/epoxy weight ratio (3, 6 and 9/100, phr), carbon nanofiller type and content on viscosity of epoxy compositions during storage at ambient temperature was evaluated. Curing process was controlled for neat and CNT or GN modified epoxy compositions (0.25-1.0 wt.% load) using differential scanning calorimetry and rheometry. Epoxy nanocomposites exhibited slightly increased glass transition temperature values (146 to 149°C) whereas tan δ and storage modulus decreased (0.30 to 0.27 and 2087 to 1070 MPa, respectively) as compared to reference material. Crosslink density regularly decreased for composites with increasing CNT content (11 094 to 7 020 mol/m3). Electrical volume resistivity of the nanocomposites was improved in case of CNT to 4•101 Ω•m and GN to 2•105 Ω•m (nanofiller content 1 wt.%). Flame retardancy was found for modified epoxy materials with as low GN and phosphorus content as 0.25 and 0.7 wt.%, respectively (increase of limiting oxygen index to 26.5%).
The detection of the early stages of ageing in an LDPE+graphite composite by comparison of dielectric responses induced by sinusoidal and triangular signals
I. Petronijevic, K. Simonovic, F. Marinkovic, J. Dojcilovic, A. S. Luyt, D. Dudic
Vol. 8., No.10., Pages 733-744, 2014
Vol. 8., No.10., Pages 733-744, 2014
This study describes the possibility of dielectric characterization of the initial stages of ageing in an low-density polyethylene (LDPE) + graphite composite, which is not possible using the standard method of dielectric spectroscopy. It is shown that the differences between the delay angles, Δφ = φTRI – φSIN, obtained using triangular and sinusoidal excitations on the composite samples, shows a maximum, and at the same time the position of this maximum shows more sensitivity to changes in the electrical properties of the material caused by ageing than other dielectric parameters. In order to clarify the applied methodology, a comparative analysis of the dielectric properties of other polymers poly(vinyl chloride) (PVC) and poly(vinyl alcohol) (PVA) and a conductive polymer composite (LDPE + carbon black) with respect to the application of sinusoidal and triangular electrical signals was carried out. Based on the presented results, we believe that the position of the peak in the frequency spectra of the difference between the delay angles obtained by using triangular and sinusoidal signals may be a suitable parameter for the dielectric characterization of polymeric materials.
For the first time, microwave assisted aniline oxidative polymerization is performed in the presence of acetic acid (CH3COOH) and ammonium hydroxide (NH4OH) at different microwave power levels. The reaction system is kept at constant temperature of 24±1°C. The products are investigated by Fourier Transform Infrared Spectroscopy (FTIR), Raman, solid-state Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectroscopies. EPR signals in polyaniline (PANI) originate from the polarons formed upon protonation and doping by acid. The microwave radiation causes an increase in the spin concentration which is slightly more evident for 8 W than for 93 W. The morphology is investigated by using scanning electron microscopy (SEM). SEM micrographs revealed the formation of nanorods (in the presence of CH3COOH) and nanospheres (in the presence of NH4OH). FTIR, Raman and solid-state NMR spectroscopies indicate the presence of PANI and aniline oligomers. X-ray Diffraction (XRD) measurements showed the presence of well-ordered structures.
Poly(ethylene-co-vinyl alcohol) of 48 mol% ethylene content was modified with N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)-isophthalamide (Nylostab SEED) to decrease the oxygen permeability of the polymer. The additive was added in a wide concentration range from 0 to 10 wt%. The structure and properties of the polymer were characterized with various methods including differential scanning calorimetry, X-ray diffraction, mechanical testing, optical measurements and oxygen permeation. Interactions were estimated by molecular modeling and infrared spectroscopy. The results showed that oxygen permeation decreased considerably when the additive was added at less than 2.0 wt% concentration. The decrease resulted from the interaction of the hydroxyl groups of the polymer and the amide groups of the additive. The dissolution of the additive in the polymer led to decreased crystallinity, but also to decreased mobility of amorphous molecules. Stiffness and strength, but also deformability increased as a result. Above 2 wt% the additive forms a separate phase leading to the deterioration of properties. The success of the approach represents a novel way to control oxygen permeation in EVOH and in other polymers with similar functional groups capable of strong interactions.
The presented research study aims to evaluate microfibers from grain by-products as a substitute for wood flour in wood-thermoplastic composites. Grain husks are an abundant and cheap source of annual, renewable raw material, which besides lignocellulose, may also contain substantial amounts of starch, proteins and fats. These grain residues may negatively affect the mechanical properties of their composites, and generate an odor when decomposition occurs at higher temperatures during plastics processing. Such odors may also be present in the end-product. In order to overcome this drawback, in this research study, a simple and effective enzymatic treatment is proposed. This environmental friendly process removed protein, starch and fats in selective manner. Treated microfibers have shown enhanced thermal stability for ca. 20°C at 1% of weight loss. This correlates with lower amount of odor emission during plastics processing as well as in the final, injection molded parts (25–65% decrease). The mechanical properties of composites were either preserved, or slightly improved. All results were compared to standard injection molded softwood WPC.
Multi-branched poly(D-lactide)s (mbPDLAs) with various structures are synthesized via ring-opening polymerization by using polyglycidol (PG) macro-initiators. Their chemical structures and thermal properties are controlled by adjusting feed ratios of D-lactide (DLA) and PG. The materials are blended with commercial linear poly(L-lactide)(l-PLLA) to form a stereocomplex structure. Effects of mbPDLAs structures and l-PLLA/mbPDLA ratios on the blends’ thermal, mechanical, and rheological properties are evaluated. Mechanical properties of the stereocomplex blends, especially elongation at break and toughness, are dependent on the blend compositions, in which a 90:10 ratio exhibits the most desirable properties. The material also exhibits the lowest complex viscosity, which provides easy processing conditions. This is achieved by the incorporation of copolymers with multi-branched structures and an ability to form a much stronger stereocomplex structure.